The LSST Camera for the Vera C. Rubin Observatory has been constructed at SLAC National Accelerator Laboratory. The Camera covers a 3.5-degree field of view with 3.2 gigapixels. The goal of the LSST survey is to provide a well-understood astronomical source catalog to the community. The LSST Camera’s focal plane is populated by 189 sensors on the science focal plane that are a combination of E2V CCD250 and ITL STA3800 deep-depletion, back-illuminated devices, accompanying eight guide sensors, and four wavefront sensors. Nine science sensors are grouped as a ”Raft” with three identical electronics boards (REBs), each operating three sensors. The REB can change the operating voltages and CCD clock, allowing operation of sensors from two different vendors in the same focal plane. We conducted phased electro-optical testing campaigns to characterize and optimize the sensor performance in the construction phase. We collected images with the focal plane illuminated by flat illuminators and some specialty projectors to produce structured images. During these tests, we found some performance issues in noise, bias stability, gain stability, image persistence, and distortion in flat images, including ”tearing”. To mitigate those non-idealities, we attempted different clocking and operation voltages and switching from unipolar voltages to bipolar voltages in parallel clock rails for E2V devices. We describe the details and the results of the optimizations.
The LSST Camera is the sole instrument for the Vera C. Rubin Observatory and consists of a 3.2 gigapixel focal plane mosaic with in-vacuum controllers, dedicated guider and wavefront CCDs, a three-element corrector whose largest lens is 1.55m in diameter, six optical interference filters covering a 320–1050 nm bandpass with an out-of-plane filter exchange mechanism, and camera slow control and data acquisition systems capable of digitizing each image in 2 seconds. In this paper, we describe the verification testing program performed throughout the Camera integration and results from characterization of the Camera’s performance. These include an electro-optical testing program, measurement of the focal plane height and optical alignment, and integrated functional testing of the Camera’s major mechanisms: shutter, filter exchange system and refrigeration systems. The Camera is due to be shipped to the Rubin Observatory in 2024, and plans for its commissioning on Cerro Pachon are briefly described.
The LSST Camera is a complex, highly integrated instrument for the Vera C. Rubin Observatory. Now that the assembly is complete, we present the highlights of the LSST Camera assembly: successful installation of all Raft Tower Modules (RTM) into the cryostat, integration of the world’s largest lens with the camera body, and successful integration and testing of the shutter and filter exchange systems. While the integration of the LSST Camera is a story of success, there were challenges faced along the way which we present: component failures, late design changes, and facility infrastructure issues.
Electro-optical testing and characterization of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Camera focal plane, consisting of 205 charge-coupled devices (CCDs) arranged into 21 stand-alone Raft Tower Modules (RTMs) and 4 Corner Raft Tower Modules (CRTMs), is currently being performed at the SLAC National Accelerator Laboratory. Testing of the camera sensors is performed using a set of custom-built optical projectors, designed to illuminate the full focal plane or specific regions of the focal plane with a series of light illumination patterns: the crosstalk projector, the flat illuminator projector, and the spot grid projector. In addition to measurements of crosstalk, linearity and full well, the ability to project realistically-sized sources, using the spot grid projector, makes possible unique measurements of instrumental signatures such as deferred charge distortions, astrometric shifts due to sensor effects, and the brighter-fatter effect, prior to camera first light. Here we present the optical projector designs and usage, the electro-optical measurements and how these results have been used in testing and improving the LSST Camera instrumental signature removal algorithms.
The Integration and Verification Testing and characterization of the expected performance of the Large Synoptic Survey Telescope (LSST) Camera is described. The LSST Camera will be the largest astronomical camera ever constructed, featuring a 3.2 Gpixel focal plane mosaic of 189 CCDs. In this paper, we describe the verification testing program developed in parallel with the integration of the Camera, and the results from our performance characterization of the Camera. Our testing program includes electro-optical characterization and CCD height measurements of the focal plane, at several steps during integration, as well as a complete functional and characterization program for the finished focal plane. It also includes a suite of functional tests of the major Camera mechanisms: shutter, filter exchange system and thermal control. Finally, we expect to test the fully assembled Camera prior to its scheduled completion and delivery to the LSST observatory in early calendar 2021.
The 3.2 gigapixel LSST camera, an array of 189 thick fully-depleted CCDs, will repeatedly image the southern sky and accomplish a wide variety of science goals. However, its trove of tens of billions of object images implies stringent requirements on systematic biases imprinted during shift-and-stare CCD observation. In order to correct for these biases which, without correction, violate requirements on weak lensing precision, we investigate CCD systematics using both simulations of charge transport as well as with a unique bench-top optical system matched to the LSST’s fast f/1.2 beam. By illuminating single CCDs with realistic scenes of stars and galaxies and then analyzing these images with the LSST data management pipelines, we can characterize the survey’s imaging performance well before the camera’s first light. We present measurements of several CCD systematics under varying conditions in the laboratory, including the brightness-dependent broadening of star and galaxy images, charge transport anomalies in the silicon bulk as well as the edges, and serial deferred charge. Alongside these measurements, we also present the development and testing of physics-based models which inform corrections or mitigation strategies for these systematics. Optimization of the CCD survey operation under a variety of realistic observational conditions, including systematic effects from the optics, clocking, sky brightness, and image analysis, will be critical to achieve the LSST’s goals of precision astronomy and cosmology.
We describe a camera beam simulator for the LSST which is capable of illuminating a 60mm field at f/1.2 with realistic astronomical scenes, enabling studies of CCD astrometric and photometric performance. The goal is to fully simulate LSST observing, in order to characterize charge transport and other features in the thick fully-depleted CCDs and to probe low level systematics under realistic conditions. The automated system simulates the centrally obscured LSST beam and sky scenes, including the spectral shape of the night sky. The doubly telecentric design uses a nearly unit magnification design consisting of a spherical mirror, three BK7 lenses, and one beam-splitter window. To achieve the relatively large field the beam-splitter window is used twice. The motivation for this LSST beam test facility was driven by the need to fully characterize a new generation of thick fully-depleted CCDs, and assess their suitability for the broad range of science which is planned for LSST. Due to the fast beam illumination and the thick silicon design [each pixel is 10 microns wide and over 100 microns deep] at long wavelengths there can be effects of photon transport and charge transport in the high purity silicon. The focal surface covers a field more than sufficient for a 40×40mm LSST CCD. Delivered optical quality meets design goals, with 50% energy within a 5 micron circle. The tests of CCD performance are briefly described.
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